1. Field of the Invention
This invention relates generally to a semiconductor technique and more particularly to a method for forming on a semiconductor substrate a silicon-containing insulation film having high mechanical strength by using a plasma CVD (chemical vapor deposition) apparatus.
2. Description of the Related Art
As semiconductors have progressed to accommodate a demand for high speed and high density in recent years, a reduction of capacitance between lines is required to avoid signal delays in the multi-layer wiring technology field. Because a reduction in the dielectric constant of a multi-layer wiring insulation film is required in order to reduce the capacitance between lines, insulation films having low dielectric constants have been developed.
Conventionally, a silicon oxide (SiOx) film is formed by adding oxygen (O2), nitric oxide (NO) or nitrous oxide (N2O) as an oxidizing agent to a silicon source gas such as SiH4 and Si(OC2H5)4 and applying heat or plasma energy to the source gas. A dielectric constant (xcex5) of this film was approximately 4.0.
By contrast, by using a spin-coat method using inorganic silicon oxide glass (SOG) materials, a low dielectric constant insulation film having a dielectric constant (xcex5) of approximately 2.3 was formed.
By using a plasma CVD method with CxFyHz as a source gas, a low dielectric constant fluorinated amorphous carbon film having a dielectric constant (xcex5) of approximately 2.0 to 2.4 was formed. Further, by using a plasma CVD method using a silicon-containing hydrocarbon (for example, P-TMOS (phenyltrimethoxysilane) as a source gas, a low dielectric constant insulation film having a dielectric constant (xcex5) of approximately 3.1 was formed. Additionally, by using a plasma CVD method using a silicon-containing hydrocarbon having multiple alkoxy groups as a source gas, a low dielectric constant insulation film having a dielectric constant (xcex5) of approximately 2.5 was formed when optimizing film formation conditions.
However, the above-mentioned conventional approaches have the following problems:
In the case of the inorganic SOG insulation film formed by the spin-coat method, there are problems in that the materials properties are not distributed equally on a silicon substrate and that a device used for a curing process after coating the material is expensive.
In the case of the fluorinated amorphous carbon film formed by the plasma CVD method using CxFyHz as a source gas, there are problems such as low heat resistance (370xc2x0 C. or lower), poor adhesion with silicon materials, and low mechanical strength of the film formed.
Furthermore, among silicon-containing hydrocarbons, when P-TMOS is used, a polymerized oligomer cannot form a linear structure such as a siloxane polymer because P-TMOS contains three alkoxy groups. Consequently, a porous structure is not formed on a silicon substrate, and hence a dielectric constant cannot be reduced to a desired degree.
When a silicon-containing hydrocarbon containing two alkoxy groups is used, a polymerized oligomer can form a linear structure such as a siloxane polymer by optimizing film formation conditions. Consequently, a porous structure can be formed on a silicon substrate and a dielectric constant can be reduced to a desired degree. However, there are problems in that oligomers having the linear structure have weak bonding power therebetween and thus the mechanical strength of a resultant film is low.
In view of the above problems, an object of the present invention is to provide a method of forming an insulation film having a low dielectric constant and high mechanical strength. Another object of the present invention is to provide a method of forming an insulation film having a low dielectric constant without increasing device costs.
To solve the above-mentioned problems, in an embodiment of the present invention, the method of forming an insulation film having a low-dielectric constant according to the present invention comprises the following processes: A process of bringing a reaction gas comprising a silicon-containing hydrocarbon having cross-linkable groups such as multiple alkoxy groups and/or vinyl groups, a cross-linking gas, and an inert gas into a reaction chamber, a process of applying radio-frequency power by overlaying first radio-frequency power and second radio-frequency power or applying the first radio-frequency power alone for generating a plasma reaction field inside the reaction chamber, and a process of optimizing the flow rates of respective source gases and the intensity of each radio-frequency power.
As the source gas, a silicon-containing hydrocarbon having multiple cross-linkable groups is used singly or in combination with one or more other silicon-containing hydrocarbons such as those having one or more cross-linkable groups. The cross-linkable groups include, but are not limited to, alkoxy groups and/or vinyl groups. For example, if a silicon-containing hydrocarbon having no or one alkoxy group is exclusively used, a linear siloxane oligomer can be formed when supplementing oxygen using an oxygen-supplying gas as necessary. However, in that case, it is difficult to cross-link oligomers by using a cross-linking gas in order to increase mechanical strength of a resultant film. A silicon-containing hydrocarbon having no or one alkoxy group can be used in an amount less than a silicon-containing hydrocarbon having two or more alkoxy groups. In an embodiment, 10% or more (including 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%) of the gas may be a silicon-containing hydrocarbon having two alkoxy groups in order to predominantly or significantly form linear oligomers. Preferably, as the source gas, the silicon-containing hydrocarbon having multiple alkoxy groups is a linear compound such as dimethyldimethoxysilane (DM-DMOS) or 1,3-dimethoxy-tetramethyldisiloxane (DMOTMDS). A silicon-containing hydrocarbon having a cyclic main structure may be used in an amount less than a linear silicon-containing hydrocarbon. In the above, alkoxy groups include xe2x80x94OCnH2n+1 (n is an integer of 1-4). The source gas can be a compound containing vinyl groups such as 1,3-divinyltetramethyldisiloxane, and similarly to a compound having alkoxyl groups, the compound can form oligomers.
As a cross-linking gas (xe2x80x9ccross-linkerxe2x80x9d), any suitable reactive gas such as CO2, ethylene glycol, 1,2-propanediol, isopropyl alcohol (IPA), ethylene, N2 or diethyl ether can be used which can cross-link oligomers of silicon-containing hydrocarbon. For example, any suitable alcohol, ether, and/or unsaturated hydrocarbon can be used, which include an alcohol selected from the group consisting of C1-6 alkanol and C4-12 cycloalkanol, and the unsaturated hydrocarbon selected from the group consisting of C1-6 unsaturated hydrocarbon, C4-12 aromatic hydrocarbon unsaturated compounds, and C4-12 alicyclic hydrocarbon unsaturated compounds. In the foregoing, compounds having a higher number of carbon atoms include, but are not limited to: 1,4-cyclohexane diol (b.p. 150xc2x0 C./20 mm), 1,2,4-trivinylcyclohexane (b.p. 85-88xc2x0 C./20 mm), 1,4-cyclohexane dimethanol (b.p. 283xc2x0 C.), and 1,3-cyclopentane diol (80-85xc2x0 C./0.1 Torr). Further, compounds having multiple reactive groups (xe2x80x98mixedxe2x80x99 functionalities, i.e., unsaturated hydrocarbon and alcohol functionalities) can also be used as cross-linkers, which include, but are not limited to: C3-20 ether such as ethylene glycol vinyl ether H2Cxe2x95x90CHOCH2OH (b.p. 143xc2x0 C.), ethylene glycol divinyl ether H2Cxe2x95x90CHOCH2CH2OCHxe2x95x90CH2 (b.p. 125-127xc2x0 C.), and 1,4-cyclohexane dimethanol divinyl ether (b.p. 126xc2x0 C./14 mm) (H2Cxe2x95x90C(OH)xe2x80x94CH2)2xe2x80x94(CH2)6); and C5-12 cycloalkanol compounds such as 1-vinylcyclohexanol (b.p. 74xc2x0 C./19 mm). Usable reactive gases are not limited to the above and will be explained below. As an inert gas, Ar, Ne, and/or He may be used. Further, as an oxygen-supplying gas, O2, NO, O3, H2O or N2O can be included to supply oxygen in the source gas if sufficient oxygen atoms are not present in the silicon-containing hydrocarbon.
In an embodiment, by overlaying high-frequency RF power and low-frequency RF power, the cross-linking of oligomers can effectively be performed. For example, a combination of high-frequency RF power having 2 MHz or higher frequencies and low-frequency RF power having less than 2 MHz frequencies can be used. The low-frequency RF power is effective even at a low power level such as 0.5 W/cm2 or lower (including 0.2, 0.1, 0.075, 0.05, 0.025 W/cm2, and a range including any two of the foregoing). In contrast, the high-frequency RF power is applied at a high power level such as 1.5 W/cm2 or higher (including 2.0, 2.25, 2.5, 2.75, 3.0, 3.25, 3.5 W/cm2, and a range including any two of the foregoing). Such a high power level can increase the mechanical strength and deposition rate of a resultant insulation film.
According to an embodiment of the present invention, a silicon-containing insulation film having a low dielectric constant and high mechanical strength can effectively be formed by using a cross-linking gas and optimizing the flow rate of each gas and the power intensity of the radio-frequency power source(s). In the embodiment, a low-dielectric constant is achieved by formation of oligomers (e.g., siloxane polymers) composed of residues of silicon-containing hydrocarbons each having two or more alkoxy groups, and high mechanical strength is achieved by cross-linking the oligomers while maintaining a low-dielectric constant. Additionally, according to an embodiment of the present invention, an insulation film having a low-dielectric constant can easily be formed without increasing device costs.
The present invention is also drawn to a method for increasing mechanical strength of an insulation film formed on a semiconductor substrate, residing in the features described above. In an embodiment, an insulation film has a dielectric constant of 2.8 or less (including 2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1, 2.0, and a range including any two of the foregoing) and a hardness (mechanical strength) of 1.0 GPa or higher (including 1.25, 1.5, 1.75, 2.0, 2.25, 2.5, 2.75, 3.0, and a range including any two of the foregoing), depending on the type of source gas, the type of cross-linking gas, and the intensity of RF power, for example.
According to an embodiment of the present invention, a silicon-containing insulation film can be formed on a substrate, which film is a plasma polymerization product obtainable by the above-mentioned method. The plasma polymerization product has a structure where silicon-containing hydrocarbon compounds each containing plural alkoxy groups are cross-linked using a cross-linking agent selected from the group consisting of C1-6 alkanol, C1-6 ether, C1-6 unsaturated hydrocarbon, CO2, and N2. The plasma polymerization product may have a hardness of 2.5 GPa or higher and a dielectric constant of 2.8 or lower, or a hardness of 1.0 GPa or higher and a elastic modulus of 5.0 GPa or higher as well as a dielectric constant of 2.5 or lower, for example, depending on the type of source gas and cross-linking gas and the plasma polymerization conditions. In the present invention, polymerization includes oligomerization, and oligomers include structures of (M)n (M is a constituent unit, n is an integer of 2-50, including ranges of 5-30 and 10-20).
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.